Broadly speaking, polyimides have excellent electrical insulation and heat resistance properties. Polyimide films are therefore often used in the manufacture of flexible circuits. In one type of flexible circuit configuration, a thin polyimide film is laminated (directly or indirectly) to a flexible metal layer, e.g., copper foil.
The present invention is applicable to (but not limited to) any of the three common processing methods for producing such polyimide-metal laminates: (1) interposing an adhesive (or similar-type bonding type) layer between the polyimide film and the metal foil; (2) applying the metal layer directly onto a polyimide film using metal vapor deposition, metal plating, and/or the like; and (3) coating a copper foil with a polyimide precursor (e.g., an amic acid varnish, solvent-soluble polyimide varnish or similar-type chemistry), followed by a processing step to convert the precursor to a polyimide (typically done by curing).
Such laminates can have failure problems, due to poor adhesion or bonding between the polyimide layer and the metal layer. Such unwanted separation can cause a complete failure of the flexible circuit.
U.S. Pat. Nos. 5,122,563, 5,268,446, 5,478,913, 5,502,157, 5,741,585, disclose a variety of end-capping agents which can be employed with polyimide chemistry. These end-capping agents are typically used to control the viscosity of the polyamic acid, or final molecular weight of the polyimide.